Persistence Niche Research Articles

Disturbance impacts on the persistence niche of key species in the Baikiaea–Guibourtia–Pterocarpus woodlands of north-western Zimbabwe

The persistence niche (studied through different modes of regeneration) of three key species (Baikiaea plurijuga (Harms), Guibourtia coleosperma (Benth.) J. Leonard, and Pterocarpus angolensis) (DC) under five different disturbance regimes (eight years after cessation of disturbance) was studied in the Baikiaea–Guibourtia–Pterocarpus woodlands of north-western Zimbabwe. The disturbance regimes were: protected area (no disturbance); timber concession area (timber harvesting); pole and firewood harvesting sites; abandoned crop fields (after clearing for agriculture); and burnt sites (fire disturbance). Influence of stump diameter and height on regeneration factors was investigated. For each disturbance regime, six main plots (100 m × 100 m) each were systematically laid out (20 m apart) to assess regeneration. In each main plot, 10 m wide transects (5 m apart) were surveyed for seedlings and saplings of the target species (at least 100 plants of each species), resulting in 7 transects per main plot, 42 transects in each study site and 210 transects in total. Number of seedlings with or without a rootstock (i.e., just grown from seed) was recorded. Diameter of stumps was measured at the cut surface. Shoot height was measured from their origin. Each target species was associated with mode of regeneration (χ² = 27 642, p < 0.001). Most plants regenerated from root suckers. Regeneration mode is dependent on disturbance regime (B. plurijuga χ² = 225.66, p < 0.001; G. coleosperma χ² = 158.62, p < 0.001; and P. angolensis χ² = 144.01, p < 0.001). Stump diameter negatively influenced number of sprouts for B. plurijuga and P. angolensis, but positively influenced sprout height for all species. Stump height positively influenced sprout height for G. coleosperma and P. angolensis. The study therefore concludes that disturbances are necessary in facilitating sprouting and growth of suppressed shoots. Stump height and diameter influence the ability of stumps to regrow vegetatively through sprouting. Weak relationships between stump diameter/height and coppice density and shoot height suggest that there are other factors that affect vegetative regrowth through coppicing.

Frequent wildfires erode tree persistence and alter stand structure and initial composition of a fire‐tolerant sub‐alpine forest

AbstractQuestionFrequent severe wildfires have the potential to alter the structure and composition of forests in temperate biomes. While temperate forests dominated by resprouting trees are thought to be largely invulnerable to more frequent wildfires, empirical data to support this assumption are lacking. Does frequent fire erode tree persistence by increasing mortality and reducing regeneration, and what are the broader impacts on forest structure and understorey composition?LocationSub‐alpine open Eucalyptus pauciflora forests, Australian Alps, Victoria, Australia.MethodsWe examined tree persistence and understorey composition of E. pauciflora open forests that were unburned, burned once, twice or three times by high‐severity wildfires between 2003 and 2013. At each of 20 sites (five per fire frequency class) we assessed extent of top‐kill and mortality of eucalypt clumps, spatial configuration of surviving and dead clumps, densities of new and lignotuberous eucalypt seedlings, and shrub and grass cover.ResultsAt least 2 yr after the last wildfire, proportions of top‐killed E. pauciflora stems were significantly higher, and densities of live basal resprouts significantly lower, at sites burned two or three times compared to once burned or unburned sites. Clump death increased to 50% of individuals at sites burned by three short‐interval wildfires, which led to changes in live tree patchiness, as indicated by nearest‐neighbour indices. Increased tree mortality was not offset by seedling recruitment, which was significantly lower at the twice‐ and thrice‐burned sites relative to single‐burn sites – although seedling recruitment was also influenced by topography and coarse woody debris. In addition to changes in the tree layer, the prominence of understorey shrubs was substantially reduced, and the frequency of grasses markedly increased, after two, and particularly three wildfires.ConclusionsOur study provides strong empirical evidence of ecologically significant change in E. pauciflora forests after short‐interval severe wildfires, namely, erosion of the persistence niche of resprouting trees, and a shift in understorey dominance from shrubs to grasses. Our findings highlight the need to consider the impacts of compounded perturbation on forests under changing climates, including testing assumptions of long‐term persistence of resprouter‐dominated communities.

Intracellular Survival of Staphylococcus aureus in Endothelial Cells: A Matter of Growth or Persistence.

The Gram-positive human pathogen Staphylococcus aureus is a leading cause of severe bacterial infections. Recent studies have shown that various cell types could readily internalize S. aureus and infected cells have been proposed to serve as vehicle for the systemic dissemination of the pathogen. Here we focused on the intracellular behavior of the Community-Associated Methicillin-Resistant S. aureus strain USA300. Supporting earlier observations, we found that wild-type S. aureus strain USA300 persisted for longer period within endothelial cells than within macrophages and that a mutant displaying the small colony variant phenotype (ΔhemDBL) had increased intracellular persistence. Time-lapse microscopy revealed that initial persistence of wild-type bacteria in endothelial cells corresponded to distinct single cell events, ranging from active intracellular bacterial proliferation, leading to cell lysis, to non-replicating bacterial persistence even 1 week after infection. In sharp contrast, ΔhemDBL mutant bacteria were essentially non-replicating up to 10 days after infection. These findings suggest that internalization of S. aureus in endothelial cells triggers its persistence and support the notion that endothelial cells might constitute an intracellular persistence niche responsible for reported relapse of infection after antibiotic therapy.

Adaptive strategy of tree communities on an environmental harshness hinterland inselberg in Minas Gerais, Brazil

Trees and other plants growing in stressful environments can display adaptive strategies such as sprouting, which is considered to be a functional trait for the persistence niche. For example, inselbergs are rocky outcrops that impose limitations on many plant forms through selective pressure to adapt to these environments. The present research tested the hypotheses that environmental harshness enhances sprouting, and that multi-stemmed trees have different persistence dynamics compared with single-stemmed trees. We sampled vegetation in 2006 and 2011 across a soil depth gradient in three areas: riparian forest, inselbergs and a corridor between the first two areas. Trees with a diameter at the breast height (dbh) ≥5 cm were sampled, and sprouts were counted. Results showed that the inselberg had more sprouted individuals and a larger amount of sprouts than the other areas. Thus, sprouting as a survival strategy was efficient in dealing with the environmental harshness and may be considered an adaptive strategy of trees to enable them to persist in such environments. The dynamic of multi-stemmed trees differed from single-stemmed ones, and the inselberg was distinct from the other areas. Over the 5 years of the present study, sprouting proved to be a strategy of persistence in this habitat, a finding that is in accordance with our hypothesis. Factors contributing to the environmental harshness, such as soil depth, may be the cause of sprouting in trees.

Testing the Shifting Persistence Niche Concept: Plant Resprouting along Gradients of Disturbance.

Plant resprouting after disturbance confers community resilience because individuals persist through trade-offs in resources for buds versus those required to produce seeds. However, repeated disturbance may deplete bud banks, and population persistence may become increasingly reliant on regeneration from seed. Theory predicts a shift in community assemblage from species with a strategy of persistence by resprouting (persistence niche) to one of regeneration from seed (regeneration niche) as the disturbance frequency increases. We tested, for the first time, the shifting persistence niche concept in a model system at local and regional community scales using a phylogenetically diverse floristic assemblage. Persistence traits of vascular plants were modeled as a function of dry-down frequency in wetlands. Resprouting species occupying the persistence niche were more common in stable wetlands than in those more frequently disturbed by dry downs. The patterns of resprouting species in standing vegetation and in seed banks provide strong support for the shifting persistence niche model involving trade-offs between resprouting (clonality) and sexual reproduction.

Resprouting as a key functional trait: how buds, protection and resources drive persistence after fire

Resprouting as a response to disturbance is now widely recognized as a key functional trait among woody plants and as the basis for the persistence niche. However, the underlying mechanisms that define resprouting responses to disturbance are poorly conceptualized. Resprouting ability is constrained by the interaction of the disturbance regime that depletes the buds and resources needed to fund resprouting, and the environment that drives growth and resource allocation. We develop a buds-protection-resources (BPR) framework for understanding resprouting in fire-prone ecosystems, based on bud bank location, bud protection, and how buds are resourced. Using this framework we go beyond earlier emphases on basal resprouting and highlight the importance of apical, epicormic and below-ground resprouting to the persistence niche. The BPR framework provides insights into: resprouting typologies that include both fire resisters (i.e. survive fire but do not resprout) and fire resprouters; the methods by which buds escape fire effects, such as thick bark; and the predictability of community assembly of resprouting types in relation to site productivity, disturbance regime and competition. Furthermore, predicting the consequences of global change is enhanced by the BPR framework because it potentially forecasts the retention or loss of above-ground biomass.

Understanding the regeneration niche: Microsite attributes and recruitment of eucalypts in dry forests

Lack of eucalypt recruitment is a key factor in the decline of forest and woodland remnants in low rainfall agricultural regions in Australia. Key to effective management of these forests is an understanding of the requirements and conditions that promote seed germination and seedling establishment (recruitment niche) and the persistence of lignotuberous sprouts (juvenile persistence niche). Recruitment is limited by the availability of safe microsites that are suitable for the germination and establishment of seedlings. The objective of this study was to investigate the microsites of established eucalypt seedlings and lignotuberous sprouts in healthy dry forests burnt in the previous 2–6years in the Tasmanian Midlands. The recruitment niche differed significantly to the juvenile persistence niche while the juvenile persistence niche had characteristics similar to the general forest floor. Seedling microsites were characterised by: canopy gaps and ashbeds (95% of seedlings surveyed); a predominantly northerly aspect (75% of microsites); over 220° shelter with the average distance from a sheltering object being less than 30cm and 80% of seedlings being sheltered by coarse woody debris; soil that was significantly softer (3.8 vs. 5.0kgf/m2) and less water repellent (moderate vs. severe repellency) than the forest floor; and low cover of grass. These characteristics of seedling safe sites all affect moisture availability. Our results have important implications for the management of dry forest in order to facilitate eucalypt recruitment and persistence, suggesting the need for retention of coarse woody debris and the judicious use of fire.

Ecology of plant resprouting: populations to community responses in fire-prone ecosystems

Resprouting has emerged as a key functional trait in plant ecology over the past decade with more than 400 papers published since 2000 (Web of Science). This special issue of Plant Ecology brings together a set of papers that advance our understanding of this functional trait, in the quest for developing a better conceptual framework for predicting community response to disturbance. This special issue highlights current research on all aspects of the effects of fire and other disturbances on plant resprouting behaviour and the importance of the persistence niche in structuring plant communities.

Multi‐stemmed trees in montane rain forests: their frequency and demography in relation to elevation, soil nutrients and disturbance

Summary Multi‐stemmed trees are a common component of tropical and temperate montane rain forests, where they co‐occur with single‐stemmed trees. We hypothesized that multi‐stemmed architecture should predominate in less productive sites (such as at high elevation or with low soil nutrient availability) and that, because it frequently results from sprouting, it should prevail in sites that are frequently disturbed. We also hypothesized that where multi‐stemmed architecture predominates, there should be lower rates of mortality and recruitment of individuals. We tested these hypotheses using data from permanent plots in tropical montane rain forests in Jamaica (14 years) and in temperate montane rain forests in New Zealand (19 years). The frequency of multi‐stemmed trees varied across plots in both forests (4–34% in Jamaica; 0–21% in New Zealand) along gradients of elevation and soil nutrients. Analyses examined the direct and indirect effects of the various environmental variables that are likely to drive site productivity and disturbance. We showed contrasting predictors of the frequency of multi‐stemmed trees in the tropical and temperate forests. Multi‐stemmed trees in Jamaican forests were most frequent on sites of low soil nutrient status (especially available soil phosphorus, P) whereas in New Zealand they were most frequent at high elevation sites, especially towards tree line. In both forests there was no relationship between multi‐stemmed tree frequency and slope. Turnover (the mean of mortality and recruitment rates) of multi‐stemmed trees in Jamaican forests was lowest on sites of low soil nutrient status (especially available P) but was unrelated to environmental predictors in New Zealand. In both forests, turnover rates of multi‐stemmed trees overall were 60% lower than those for co‐occurring single‐stemmed trees, offering support for the hypothesis that multi‐stemmed architecture favours persistence. Synthesis. Our study demonstrates that multi‐stemmed trees can predominate in low productivity sites in montane rain forests. Their low turnover compared with co‐occurring single‐stemmed trees confirms the importance of evaluating the persistence niche as a mechanism promoting woody species coexistence in forests.

Contrasting demographics of tropical savanna and temperate forest eucalypts provide insight into how savannas and forests function. A case study using Corymbia clarksoniana from north‐eastern Australia

AbstractEucalypts (Eucalyptus spp. and Corymbia spp.) dominate many communities across Australia, including frequently burnt tropical savannas and temperate forests, which receive less frequent but more intense fires. Understanding the demographic characteristics that allow related trees to persist in tropical savannas and temperate forest ecosystems can provide insight into how savannas and forests function, including grass–tree coexistence. This study reviews differences in critical stages in the life cycle of savanna and temperate forest eucalypts, especially in relation to fire. It adds to the limited data on tropical eucalypts, by evaluating the effect of fire regimes on the population biology of Corymbia clarksoniana, a tree that dominates some tropical savannas of north‐eastern Australia. Corymbia clarksoniana displays similar demographic characteristics to other tropical savanna species, except that seedling emergence is enhanced when seed falls onto recently burnt ground during a high rainfall period. In contrast to many temperate forest eucalypts, tropical savanna eucalypts lack canopy‐stored seed banks; time annual seed fall to coincide with the onset of predictable wet season rain; have very rare seedling emergence events, including a lack of mass germination after each fire; possess an abundant sapling bank; and every tropical eucalypt species has the ability to maintain canopy structure by epicormically resprouting after all but the most intense fires. The combination of poor seedling recruitment strategies, coupled with characteristics allowing long‐term persistence of established plants, indicate tropical savanna eucalypts function through the persistence niche rather than the regeneration niche. The high rainfall‐promoted seedling emergence of C. clarksoniana and the reduction of seedling survival and sapling growth by fire, support the predictions that grass–tree coexistence in savannas is governed by rainfall limiting tree seedling recruitment and regular fires limiting the growth of juvenile trees to the canopy.

Leaf Axil Anatomy and Bud Reserves in 21 Myrtaceae Species from Northern Australia

Dormant axillary buds allow plants to repair minor damage to their canopies. In woody plants, these buds subsequently develop into epicormic structures that may allow vegetative recovery after major disturbances. They are an essential but little‐studied part of the persistence niche. We wondered what bud reserves were present in the leaf axils of northern Australian myrtaceous species, what levels of protection they have, and how this relates to the ecology of these species. Axillary buds of 21 species from 10 genera of northern Australian Myrtaceae were examined anatomically. All species possessed axillary buds in all axils examined, and accessory buds were recorded in 86% of species. The species exhibited an extremely wide range of variation—from axillary buds that consisted of only an apical dome with no leaf primordia (Calytrix exstipulata) to axils with a complex array of accessory buds and meristems located beneath the axil surface (Corymbia and Eucalyptus). The axils of the Eucalyptus and Corymbia species had a greater number of and better protected axillary buds and meristems than the other species studied, including some of their closest relatives, Arillastrum, Allosyncarpia, and Stockwellia. All investigated species had an excellent meristem reserve for recovery of photosynthetic capacity after minor canopy damage and for developing epicormic structures for sprouting after more severe damage. The complex and well‐protected axillary bud or meristem structures of Corymbia and Eucalyptus may be an important component of the success of these genera in Australia.

Evergreen broad-leaved forest in Eastern China: Its ecology and conservation and the importance of resprouting in forest restoration

Biogeographical and ecological research on the evergreen broad-leaved forests (EBLF) of the Far East, which have been reduced to remnant fragments over the historical period has, until now, been very limited. To inform forest conservation and management, this paper describes research into major plant community types and underlying environmental gradients of degraded EBLF in Eastern China and examines the importance of resprouting as a key mechanism in secondary succession following forest clearance. Species composition was described from 199 10 m × 10 m plots and analysed using two-way indicator species analysis and canonical correspondence analysis ordination. Some 22 degraded and mature forest community types were identified, while CCA indicated that a primary vegetation gradient was related to distance of sample plot from mature forest, which was closely linked to altitude and slope. The secondary gradient corresponded to successional stage and disturbance. The roles of resprouting and reseeding characteristics in forest regeneration were researched firstly by 10 m × 10 m plots from selected TWINSPAN groups and secondly by 20 m × 20 m plots in representative areas of forest at 1, 20, 43 and 60 years and in an area of mature forest. The importance of resprouting in the regeneration of many EBLF tree and shrub species is demonstrated, a process linked to ideas of the persistence niche. Existing remnant forests should be conserved but forest restoration is also essential and will benefit from understanding of the importance of tree/shrub resprouting, as well as seedling recruitment in forest regeneration. Further work is required on seedbanks, germination success and both inter- and intra-specific competition within Chinese EBLF to assist with successful forest conservation and management.

The persistence niche: what makes it and what breaks it for two fire-prone plant species

Persistence niches are expected to favour qualitatively different plant life histories compared with regeneration niches. In fire-prone habitats, for example, resprouting plants may be expected to exploit persistence niches, whereas obligate-seeders by definition exploit regeneration niches. Resprouter life histories should be typified by high rates of survival, which may be offset by relatively low rates of growth and reproduction. This combination of characters is expected to result from trade-offs in resource allocation and because the longevity of individual plants should buffer their populations against the effects of recruitment failure. We asked whether two resprouting perennial shrubs, Epacris barbata Melville and Xanthorrhoea resinifera (Sol. Ex Kite) E.C.Nelson &amp; D.J.Bedford, exhibited the life-history character combinations that are expected for species exploiting a persistence niche. We also investigated how a change in habitat suitability caused by the invasion of a root pathogen may limit the ability of these species to occupy persistence niches. Demographic censuses of several years’ duration in two populations of each species yielded estimates of vital rates that were consistent with the life-history profile expected for a persistence niche. Rates of background survival were high and rates of fire-related mortality were low in both species. As expected, these were associated with low rates of growth and seedling establishment, although rates of seed production and viability were relatively high in both species. The importance of survival was confirmed by stochastic population models, which showed that population viability was more sensitive to decreases in survival of mature plants and increases in fire mortality of established plants than to changes in other vital rates. Seedling growth rates were also relatively important in E. barbata. Populations of both species that had been infected by root rot disease, Phytophthora cinnamomi, had substantially reduced survival rates and, consequently, reduced population viability. These effects were more extreme in E. barbata than in X. resinifera. We conclude that processes that reduce survival, such as disease infection and habitat loss, rather than processes that impede seed production and recruitment mediate the persistence niche. However, we discuss the possibility that this dependency might be mitigated by high fecundity if infrequent conditions that permit large recruitment events have so far eluded detection.